Method for Quantifying the Amount of Ammonium Bicarbonate in a Solid Sample of Ammonium Bicarbonate

ABSTRACT

A method for quantifying the amount of ammonium bicarbonate in a solid sample of ammonium carbamate is provided. The method includes measuring the FT-IR spectrum of the sample, calculating the IR band maximum for a first band that is common to ammonium carbamate and ammonium bicarbonate and for a second band that is unique to ammonium carbamate, calculating a ratio of the maximum of the second band to the maximum of the first band, and calculating the concentration of ammonium bicarbonate in the sample from a calibration curve.

CROSS-REFERENCE TO RELATED APPLICATION

Reference is made to U.S. Provisional Patent Application Ser. No.62/013,154, filed Jun. 17, 2014 and entitled METHOD FOR QUANTIFYING THEAMOUNT OF AMMONIUM BICARBONATE IN A SOLID SAMPLE OF AMMONIUM CARBAMATE,the disclosure of which is hereby incorporated by reference and priorityof which is hereby claimed pursuant to 37 CFR 1.78(a)(4) and (5)(i).

FIELD OF THE INVENTION

The present invention relates to a method for quantifying the amount ofammonium bicarbonate in a solid sample of ammonium carbamate.

BACKGROUND OF THE INVENTION

Various techniques are known for quantifying ammonium carbonate,ammonium bicarbonate and ammonium carbamate in a sample.

SUMMARY OF THE INVENTION

The present invention seeks to provide a method for quantifying theamount of ammonium bicarbonate in a solid sample of ammonium carbamate.

There is thus provided in accordance with a preferred embodiment of thepresent invention a method for quantifying the amount of ammoniumbicarbonate in a solid sample of ammonium carbamate including:

a. measuring the FT-IR spectrum of the sample;

b. calculating the IR band maximum for a first band that is common toammonium carbamate and ammonium bicarbonate and for a second band thatis unique to ammonium carbamate;

c. calculating a ratio of the maximum of the second band to the maximumof the first band; and

d. calculating the concentration of ammonium bicarbonate in the samplefrom a calibration curve relating the concentration to the ratio.

In a preferred embodiment of the present invention, the measuring theFT-IR spectrum of the sample includes measuring the attenuated totalreflectance of the solid sample when pressed against a crystal.Preferably, the crystal is a diamond crystal.

In a preferred embodiment of the present invention, the first band is2781-2875 cm⁻¹. In another preferred embodiment of the presentinvention, the second band is 3423-3500 cm⁻¹. In a preferred embodimentof the present invention, the maximum for a first band and the maximumfor a second band are each corrected by subtracting the averageabsorbance in a background band. Preferably, the background band is3870-3999 cm⁻¹.

In a preferred embodiment of the present invention, the calibrationcurve is constructed by:

a. preparing a plurality of calibration samples including knownquantities of ammonium carbamate and ammonium bicarbonate;

b. measuring the FT-IR spectrum of each of the plurality of calibrationsamples;

c. for each of the plurality of calibration samples, calculating the IRband maximum for the first band and for the second band;

d. for each of the plurality of calibration samples, calculating a ratioof the maximum of the second band to the maximum of the first band; and

e. creating a regression curve of the bicarbonate composition as afunction of the ratio.

Preferably, the regression curve is a second-order polynomial curve. Ina preferred embodiment of the present invention, the regressioncoefficient of the regression curve is at least 0.99. Preferably, thecalibration curve is reconstructed once a month.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawing in which:

FIG. 1 is the FT-IR absorbance spectrum of ammonium carbamate; and

FIG. 2 is the FT-IR absorbance spectrum of ammonium bicarbonate.

DETAILED DESCRIPTION OF THE INVENTION

Ammonium carbamate can be formed from the reaction of ammonia withcarbon dioxide according to the following reaction:

CO₂+2NH₃→NH₂COONH₄

Ammonium carbamate is an important intermediate in the production ofurea, which is used as a nitrogen-releasing fertilizer, among otheruses. The production of urea occurs according to the following reaction:

NH₂COONH₄→(NH₂)₂CO+H₂O

The major impurity in ammonium carbamate is ammonium bicarbonate, whichis formed by the reaction of ammonium carbamate with water according tothe following reaction:

NH₂COOHN₄+H₂O→NH₄CO₃H+NH₃

Accordingly, it is important to be able to determine the purity ofammonium carbamate, and in particular to quantify the amount of ammoniumbicarbonate in ammonium carbamate.

Various methods are known for identifying and quantifying ammoniumcarbamate, ammonium carbonate and ammonium bicarbonate in a sample.Burrows et al., J. Am. Chem. Soc. (1912), 34(8):993-995 discloses amethod of separating ammonium carbamate from ammonium carbonate byselective precipitation of carbonate with barium salts. The separatedfractions are titrated with hydrochloric acid. Lugowska, Zeszyty NaukowePolitechniki Slaskiej, Chemia (1972), 60:29-37 discloses a method ofseparation carbamate from carbonate and bicarbonate by dissolution ofthe sample in acetone. Carbamate is soluble in acetone while carbonateand bicarbonate are insoluble. After separation, both fractions aretitrated with perchloric acid.

Byun, Kongop Hwahak (1994), 5(4):657-661 discloses a method ofquantifying ammonium carbamate and urea in a sample using IRspectroscopy. Absorption peaks in the near IR (NIR) range were used todistinguish between ammonium carbamate and urea. In this method, ammoniais used to inhibit the decomposition of ammonium carbamate to ammoniumbicarbonate.

Meng et al., Anal. Chem. (2005), 77(18): 5947-5952 discloses a methodfor measuring the purity of a sample containing mainly ammoniumbicarbonate using a combination of NIR spectroscopy and elementalanalysis. It is also disclosed that ammonium carbamate can bequalitatively identified using FT-IR (Fourier transform infrared)spectroscopy.

Mani et al., Green Chem. (2006), 8:995-1000 discloses a method ofdetermining the relative concentrations of carbamate, carbonate andbicarbonate using ¹³C NMR. There do not appear to be any known methodsfor quantifying ammonium bicarbonate in a solid sample of ammoniumcarbamate using FT-IR spectroscopy.

In accordance with a first embodiment of the present invention, there isprovided a method for quantifying the amount of ammonium bicarbonate ina solid sample of ammonium carbamate comprising:

-   -   a. measuring the FT-IR spectrum of the sample;    -   b. calculating the IR band maximum for a first band that is        common to ammonium carbamate and ammonium bicarbonate and for a        second band that is unique to ammonium carbamate;    -   c. calculating a ratio of the maximum of the second band to the        maximum of the first band; and    -   d. calculating the concentration of ammonium bicarbonate in the        sample from a calibration curve relating said concentration to        said ratio.

In one embodiment, the FT-IR spectrum is measured using a Nicolet® 6700FT-IR spectrometer (Thermo Fisher Scientific, Waltham, Mass. USA). FT-IRspectra are preferably collected directly from solid, homogenizedsamples without any additional sample preparation using the ATR(Attenuated Total Reflectance) technique. The samples are preferablypressed against a diamond ATR crystal.

Preferably, the spectral data are recorded in the mid infrared range(4000-400 cm⁻¹). The sampling depth is typically in the range of 0.3-3run. A background spectrum is preferably recorded against a cleandiamond ATR crystal prior to each sample measurement.

FIG. 1 shows the FT-IR spectrum of high-purity ammonium carbamate (BASF,Ludwigshafen, Germany). FIG. 2 shows the FT-IR spectrum of reagent gradeammonium bicarbonate. It can be seen from these two spectra thatammonium carbamate has an absorption band centered at 3465 cm⁻¹, whileammonium bicarbonate has no absorption band at that wavenumber.

In one embodiment, the first band common to ammonium carbamate andammonium bicarbonate is 2781-2875 cm⁻¹. The maximum absorbance in thisband is preferably corrected for background noise by subtracting theaverage absorbance in the band 3870-3999 cm⁻¹. The resulting value iscalled value A.

In one embodiment, the second band unique to ammonium carbamate that isnot present in the FT-IR spectrum of ammonium bicarbonate is 3423-3500cm⁻¹. The maximum absorbance in this band is preferably corrected forbackground noise by subtracting the average absorbance in the band3870-3999 cm⁻¹. The resulting value is called value B. These bands areshown in FIGS. 1 and 2. The ratio B/A is called value C.

The calibration curve is preferably constructed by:

-   -   e. preparing a plurality of calibration samples comprising known        quantities of ammonium carbamate and ammonium bicarbonate;    -   f. measuring the FT-IR spectrum of each of the calibration        samples;    -   g. calculating the value C for each of the calibration samples;        and    -   h. creating a regression curve of the bicarbonate composition as        a function of the value C.

In one embodiment, the calibration samples are prepared from high purityammonium carbamate and reagent grade ammonium bicarbonate. Thecalibration samples also preferably include samples of pure ammoniumcarbamate, pure ammonium bicarbonate and pure commercial ammoniumcarbonate. Commercial ammonium carbonate is a 1:1 molar ratio ofammonium carbamate and ammonium bicarbonate. Since it is expected thatthe levels of bicarbonate impurities in ammonium carbamate will be low,the calibration samples preferably include several samples at the lowend of the bicarbonate concentration range, such as 1%, 5%, 10%, 15% and20%.

The regression curve can be any curve that fits the calibration data. Ina preferred embodiment, the calibration curve is a second-orderpolynomial curve. Preferably the regression coefficient R is at least0.99. Preferably, the FT-IR instrument is recalibrated once a month.Since bicarbonate is the major impurity in the sample, the sample puritycan be estimated as 100%—bicarbonate concentration (%).

EXAMPLES Example 1

Calibration samples were prepared by mixing different amounts ofammonium bicarbonate (>99%, BDH, Radnor, Pa. USA) with high purityammonium carbamate (BASF. Ludwigshafen, Germany) as detailed in Table 1.In addition, a sample of pure commercial ammonium carbonate (Merck,Darmastadt, Germany), which is a 1:1 molar ratio of ammonium bicarbonateand ammonium carbamate, was taken for the calibration. Two samples foreach concentration were used. All calibration samples were homogenizedwith an MM 400 ball mill (Retsch, Haan, Germany) for 1 minute at 30 Hzprior to FT-IR analysis.

TABLE 1 FT-IR calibration samples. Amount of ammonium Amount of ammoniumcarbamate, w/w (%) bicarbonate, w/w (%) 100 0 99 1 95 5 90 10 85 15 8020 0 100

FT-IR spectra were collected directly from the homogenized sampleswithout any additional sample preparation using the ATR technique. Adiamond ATR crystal was utilized. The spectral data were recorded in themid infrared range (4000-400 cm⁻¹) using a resolution of 4 cm⁻¹ and 120scans. A background spectrum was recorded against a clean diamond ATRcrystal prior to each sample measurement.

For each spectrum the IR band maximum between 2781-2875 cm⁻¹ was takenand the average of the background signals in the spectral region of3870-3999 cm⁻¹ was subtracted therefrom to provide value A. In addition,the IR band maximum between 3423-3500 cm⁻¹ was taken and the average ofthe background signals in the spectral region of 3870-3999 cm⁻¹ wassubtracted therefrom to provide value B. The value B/A is the value C.

The ammonium bicarbonate concentration was plotted as a function of theC value. The data were fit to regression curves. Polynomial regressiongave the best fit, with a regression constant of 0.9932 and a formula

y=1.774C²−2.6565C+0.9963

wherein y is the ammonium bicarbonate concentration (w/w).

The FT-IR spectra of two samples from each of two industrial batches ofammonium carbamate were measured and the C values calculated as for thecalibration samples. The bicarbonate concentrations calculated from thecalibration curve and the sample purities are reported in Table 2.

TABLE 2 Ammonium bicarbonate concentration in industrial samples ofcarbamate Batch C value Bicarbonate concentration (%) Sample purity (%)1 0.528281 8 92 1 0.544591 7 93 2 0.585135 5 95 2 0.582629 5 95

Example 2

A calibration curve was prepared as described in Example 1. Polynomialregression gave the best fit, with a regression constant of 0.9944 and aformula

y=1.6588C²−2.6148C+1.0215

wherein y is the ammonium bicarbonate concentration (w/w).

The FT-IR spectra of industrial batches of ammonium carbamate from threedifferent producers were measured and the C values calculated as for thecalibration samples. The bicarbonate concentrations calculated from thecalibration curve and the sample purities are reported in Table 3.

TABLE 3 Ammonium bicarbonate concentration in industrial samples ofcarbamate Batch C value Bicarbonate concentration (%) Sample purity (%)D1 0.677193 1 99 D1 0.648803 2 98 D2 0.596883 5 95 D2 0.58691 6 94 D30.595691 5 95 D3 0.610847 4 96 D4 0.603204 5 95 D4 0.640028 3 97 D50.558668 8 92 D5 0.51993 11 89 S1 0.507155 12 88 S1 0.493468 14 86 S20.590611 6 94 S2 0.616511 4 96 S3 0.629779 3 97 S3 0.640886 3 97 S40.449107 18 82 S4 0.468347 16 84 S5 0.506865 12 88 S5 0.506601 12 88 B10.639531 3 97 B1 0.670934 1 99 B2 0.671199 1 99 B2 0.641169 3 97 B30.621668 4 96 B3 0.635751 3 97 B4 0.595292 5 95 B4 0.641512 3 97 B50.63316 3 97 B5 0.673346 1 99

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather the scope of the present inventionincludes both combinations and subcombinations of various featuresdescribed hereinabove as well as modifications thereof which would occurto a person of skill in the art upon reading the foregoing descriptionand which are not in the prior art.

1-11. (canceled)
 12. A solid sample of ammonium carbamate comprising aknown amount of ammonium bicarbonate, the known amount determined by: a)measuring an FT-IR spectrum of said sample; b) calculating an IR bandmaximum for a first band that is common to ammonium carbamate andammonium bicarbonate and for a second band that is unique to ammoniumcarbamate; c) calculating a ratio of said maximum of said second band tosaid maximum of said first band; and d) calculating a concentration ofammonium bicarbonate in said sample from a calibration curve relatingsaid concentration to said ratio.
 13. The solid sample according toclaim 12, wherein said measuring the FT-IR spectrum of said samplecomprises measuring an attenuated total reflectance of the solid samplewhen pressed against a crystal.
 14. The solid sample according to claim13, wherein said crystal is a diamond crystal.
 15. The solid sampleaccording to claim 12, wherein said first band is 2781-2875 cm⁻¹. 16.The solid sample according to claim 12, wherein said second band is3423-3500 cm⁻¹.
 17. The solid sample according to claim 12, wherein saidmaximum for a first band and said maximum for a second band are eachcorrected by subtracting an average absorbance in a background band. 18.The solid sample according to claim 17, wherein said background band is3870-3999 cm⁻¹.
 19. The solid sample according to claim 12, wherein saidcalibration curve is constructed by: a) preparing a plurality ofcalibration samples comprising known quantities of ammonium carbamateand ammonium bicarbonate; b) measuring an FT-IR spectrum of each of saidplurality of calibration samples; c) for each of said plurality ofcalibration samples, calculating an IR band maximum for said first bandand for said second band; d) for each of said plurality of calibrationsamples, calculating a ratio of said maximum of said second band to saidmaximum of said first band; and e) creating a regression curve ofbicarbonate composition as a function of said ratio.
 20. The solidsample according to claim 19, wherein said regression curve is asecond-order polynomial curve.
 21. The solid sample according to claim19, wherein a regression coefficient of said regression curve is atleast 0.99.
 22. A method for quantifying an amount of ammoniumbicarbonate in a solid sample of ammonium carbamate comprising: a.measuring an FT-IR spectrum of said sample; b. calculating an IR bandmaximum for a first band that is common to ammonium carbamate andammonium bicarbonate and for a second band that is unique to ammoniumcarbamate; c. calculating a ratio of said maximum of said second band tosaid maximum of said first band; and d. calculating a concentration ofammonium bicarbonate in said sample from a calibration curve relatingsaid concentration to said ratio.
 23. The method according to claim 22,wherein said measuring the FT-IR spectrum of said sample comprisesmeasuring an attenuated total reflectance of the solid sample whenpressed against a crystal.
 24. The method according to claim 23, whereinsaid crystal is a diamond crystal.
 25. The method according to claim 22,wherein said first band is 2781-2875 cm⁻¹.
 26. The method according toclaim 22, wherein said second band is 3423-3500 cm⁻¹.
 27. The methodaccording to claim 22, wherein said maximum for a first band and saidmaximum for a second band are each corrected by subtracting an averageabsorbance in a background band.
 28. The method according to claim 27,wherein said background band is 3870-3999 cm⁻¹.
 29. The method accordingto claim 22, wherein said calibration curve is constructed by: a)preparing a plurality of calibration samples comprising known quantitiesof ammonium carbamate and ammonium bicarbonate; b) measuring an FT-IRspectrum of each of said plurality of calibration samples; c) for eachof said plurality of calibration samples, calculating an IR band maximumfor said first band and for said second band; d) for each of saidplurality of calibration samples, calculating a ratio of said maximum ofsaid second band to said maximum of said first band; and e) creating aregression curve of bicarbonate composition as a function of said ratio.30. The method according to claim 29, wherein said regression curve is asecond-order polynomial curve.
 31. The method according to claim 29,wherein a regression coefficient of said regression curve is at least0.99.